Nowadays, motor vehicles are usually powered by combustion machines in which fuel is burned and the thermal energy released in this process is partially converted into mechanical work. The efficiency of reciprocating combustion machines, which are almost exclusively the only means employed to power motor vehicles, amounts to only about one-third of the primary energy used. Consequently, two-thirds of the thermal energy released during the combustion is waste heat that is discharged to the environment as lost heat, either via the engine cooling system or via the exhaust gas system.
The utilization of this waste heat constitutes a way to increase the overall efficiency of the drive unit of the vehicle and consequently to lower the consumption of fuel.
German patent application DE 10 2008 028 467 A1 describes a device for the utilization of waste heat from an internal combustion engine. For this purpose, a first heat exchanger, namely, the evaporator, of a steam cyclic device is integrated into the exhaust gas system of the internal combustion engine. The thermal energy transferred in the heat exchanger from the exhaust gas to a working medium of the steam cyclic device is partially converted in an expansion device into mechanical energy that can be used, for instance, to assist the drive of a motor vehicle or to generate electrical energy. Downstream from the expansion device, the working medium is cooled in a second heat exchanger, namely, the condenser, a process in which it condenses. A feed pump increases the pressure of the working medium and feeds it to the evaporator.
The condenser is cooled by the coolant of a coolant circuit of the internal combustion engine. The return line of the coolant from the condenser can be switched by means of a three-way valve in such a way that the coolant is fed into the engine coolant circuit either upstream from the main cooler of the motor vehicle or upstream from the internal combustion engine. As a result, the waste heat that the coolant absorbs in the condenser can be used during a warm-up phase of the internal combustion engine in order to more quickly warm up the coolant. Here, in a generally known manner, it is provided for the engine cooler to be bypassed via a bypass line in order to avoid an undesired cooling of the coolant during the warm-up phase. By using the waste heat that has accumulated in the condenser, the internal combustion engine more quickly reaches its operating temperature, a process that is associated with a low fuel consumption and few emissions during the warm-up phase. After the operating temperature has been reached, the coolant that has been warmed up by the condenser can be fed directly into the engine coolant circuit upstream from the engine cooler. As a result, it can be prevented that waste heat from the condenser reduces the cooling capacity available for the internal combustion engine. On the other hand, however, it can also be provided that coolant exiting from the condenser is fed into the engine coolant circuit directly upstream from the internal combustion engine once the operating temperature has been reached. This can be provided if the engine outlet temperature of the coolant falls below a setpoint value, which can happen, for example, when the motor vehicle has been driven downhill for a prolonged period of time. Moreover, then the heating capacity of an interior heater that is based on the exchange of heat with the engine coolant circuit can be improved.
It is likewise a known procedure to integrate heat storage tanks, especially latent or chemical heat storage tanks, into the engine coolant circuit (see, for example, German patent application DE 10 2008 013 650 A1). During the operation of the internal combustion engine, once the operating temperature has been reached, these heat storage tanks absorb heat from the exhaust gas and temporarily store it. After a subsequent cold start of the combustion machine, the heat storage tanks once again release the stored heat to the engine coolant circuit, as a result of which the warm-up phase of the combustion machine is shortened.
It is also a known procedure to combine both of these measures in a combustion machine, whereby there is no interaction between the steam cycle and the heat storage tank. The heat storage tank here is a pure cold-start measure, whereas the steam cycle only becomes active once the operating temperature has been reached. A drawback of these systems is that the evaporator as well as the heat storage tank are both integrated into the exhaust gas system and they each have to have a bypass line in order to allow them to be bypassed so that, when they are not being used, an increase in the exhaust gas counter-pressure caused by the evaporator and the heat storage tank can be avoided.
An alternative combination of a heat storage tank with a steam cycle for the utilization of waste heat from the exhaust gas is known from German patent application DE 10 2007 033 611 A1. In this document, during the operating states of the internal combustion engine in which the thermal energy contained in the exhaust gas is greater than the maximum amount of thermal energy in the evaporator that can be transferred to the working medium, the use of appropriately configured bypass lines and valves is provided so as to divert part of the exhaust gas flow to the heat storage tank in order to “charge” it, insofar as it is not already completely “charged”. On the other hand, during the operating states of the internal combustion engine in which the thermal energy of the exhaust gas is less than the thermal energy that can be converted in the evaporator, it is provided that the exhaust gas is first fed through the heat storage tank in order to warm up the exhaust gas, as a result of which the steam cycle can then be carried out more effectively. In order to prevent this from causing an impermissible cooling of the exhaust gas treatment devices of the exhaust gas system, the heat storage tank as well as the evaporator are integrated into the exhaust gas downstream from these exhaust gas treatment devices.
A combustion machine that is functionally similar and that has a steam cyclic device is known from German patent application DE 10 2009 035 522 A1. There, a steam reservoir is integrated into the steam cyclic device, so that the steam can be temporarily stored in this steam reservoir during times when it has been generated but cannot be utilized. The stored steam is fed back into the steam cycle again when more steam can be converted than can be generated through the utilization of exhaust gas heat.
Another alternative combination of a heat storage tank with a steam cycle for the utilization of waste heat from the exhaust gas is known from German patent application DE 10 2011 076 054 A1. For this purpose, it is provided for the heat storage tank to be integrated into the steam cycle and for it to be impinged with the working medium of the steam cycle. Here, it is provided that a supply line to the heat storage tank is integrated into the steam cycle between the evaporator and the expansion device, while a return line is integrated into the steam cycle between the expansion device and the condenser. Fundamentally, this means that the heat transferred from the heat storage tank to the working medium after a cold start of the combustion machine should be transferred in the evaporator to the exhaust gas, as a result of which the heating time for exhaust gas treatment devices arranged in the exhaust gas system downstream from the evaporator can be shortened. Moreover, German patent application DE 10 2011 076 054 A1 discloses that, as an alternative or in addition to warming up the exhaust gas during the warm-up phase, it can also be provided that the waste heat from the condenser shortens the warm-up phase of the combustion machine itself in that the condenser is cooled by the coolant of the engine coolant circuit. When the working medium is utilized in such a way, it can also be provided for the pressure of the working medium to be adjusted by means of a pressure adjuster so that the steam phase is temporarily eliminated and the working medium circulates in the liquid state throughout the entire circuit.
Before the backdrop of this state of the art, the invention was based on the objective of putting forward an improved integration of a steam cycle in a drive unit for a motor vehicle.